Scientists now have a fairly good understanding of how the plates move and how such
movements relate to earthquake activity. Most movement occurs along narrow zones
between plates where the results of plate-tectonic forces are most evident.
There are four types of plate boundaries:
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
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Divergent boundaries -- where new crust is generated as the plates pull away from
each other.
Convergent boundaries -- where crust is destroyed as one plate dives under
another.
Transform boundaries -- where crust is neither produced nor destroyed as the
plates slide horizontally past each other.
Plate boundary zones -- broad belts in which boundaries are not well defined and
the effects of plate interaction are unclear.
Illustration of the Main Types of Plate Boundaries [55 k]
Divergent boundaries
Divergent boundaries occur along spreading centers where plates are moving apart and
new crust is created by magma pushing up from the mantle. Picture two giant conveyor
belts, facing each other but slowly moving in opposite directions as they transport newly
formed oceanic crust away from the ridge crest.
Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This
submerged mountain range, which extends from the Arctic Ocean to beyond the southern
tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the
Earth. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters
per year (cm/yr), or 25 km in a million years. This rate may seem slow by human
standards, but because this process has been going on for millions of years, it has resulted
in plate movement of thousands of kilometers. Seafloor spreading over the past 100 to
200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water
between the continents of Europe, Africa, and the Americas into the vast ocean that exists
today.
Mid-Atlantic Ridge [26 k]
The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers scientists
a natural laboratory for studying on land the processes also occurring along the
submerged parts of a spreading ridge. Iceland is splitting along the spreading center
between the North American and Eurasian Plates, as North America moves westward
relative to Eurasia.
Map showing the Mid-Atlantic Ridge splitting Iceland and separating the North
American and Eurasian Plates. The map also shows Reykjavik, the capital of Iceland, the
Thingvellir area, and the locations of some of Iceland's active volcanoes (red triangles),
including Krafla.
The consequences of plate movement are easy to see around Krafla Volcano, in the
northeastern part of Iceland. Here, existing ground cracks have widened and new ones
appear every few months. From 1975 to 1984, numerous episodes of rifting (surface
cracking) took place along the Krafla fissure zone. Some of these rifting events were
accompanied by volcanic activity; the ground would gradually rise 1-2 m before abruptly
dropping, signalling an impending eruption. Between 1975 and 1984, the displacements
caused by rifting totalled about 7 m.
Lava Fountains, Krafla Volcano [35 k]
Thingvellir Fissure Zone, Iceland [80 k]
In East Africa, spreading processes have already torn Saudi Arabia away from the rest of
the African continent, forming the Red Sea. The actively splitting African Plate and the
Arabian Plate meet in what geologists call a triple junction, where the Red Sea meets the
Gulf of Aden. A new spreading center may be developing under Africa along the East
African Rift Zone. When the continental crust stretches beyond its limits, tension cracks
begin to appear on the Earth's surface. Magma rises and squeezes through the widening
cracks, sometimes to erupt and form volcanoes. The rising magma, whether or not it
erupts, puts more pressure on the crust to produce additional fractures and, ultimately, the
rift zone.
Historically Active Volcanoes, East Africa [38 k]
East Africa may be the site of the Earth's next major ocean. Plate interactions in the
region provide scientists an opportunity to study first hand how the Atlantic may have
begun to form about 200 million years ago. Geologists believe that, if spreading
continues, the three plates that meet at the edge of the present-day African continent will
separate completely, allowing the Indian Ocean to flood the area and making the
easternmost corner of Africa (the Horn of Africa) a large island.
Summit Crater of 'Erta 'Ale [55 k]
Oldoinyo Lengai, East African Rift Zone [38 k]
Convergent boundaries
The size of the Earth has not changed significantly during the past 600 million years, and
very likely not since shortly after its formation 4.6 billion years ago. The Earth's
unchanging size implies that the crust must be destroyed at about the same rate as it is
being created, as Harry Hess surmised. Such destruction (recycling) of crust takes place
along convergent boundaries where plates are moving toward each other, and sometimes
one plate sinks (is subducted) under another. The location where sinking of a plate occurs
is called a subduction zone.
The type of convergence -- called by some a very slow "collision" -- that takes place
between plates depends on the kind of lithosphere involved. Convergence can occur
between an oceanic and a largely continental plate, or between two largely oceanic plates,
or between two largely continental plates.
Oceanic-continental convergence
If by magic we could pull a plug and drain the Pacific Ocean, we would see a most
amazing sight -- a number of long narrow, curving trenches thousands of kilometers long
and 8 to 10 km deep cutting into the ocean floor. Trenches are the deepest parts of the
ocean floor and are created by subduction.
Off the coast of South America along the Peru-Chile trench, the oceanic Nazca Plate is
pushing into and being subducted under the continental part of the South American Plate.
In turn, the overriding South American Plate is being lifted up, creating the towering
Andes mountains, the backbone of the continent. Strong, destructive earthquakes and the
rapid uplift of mountain ranges are common in this region. Even though the Nazca Plate
as a whole is sinking smoothly and continuously into the trench, the deepest part of the
subducting plate breaks into smaller pieces that become locked in place for long periods
of time before suddenly moving to generate large earthquakes. Such earthquakes are
often accompanied by uplift of the land by as much as a few meters.
Convergence of the Nazca and South American Plates [65 k]
On 9 June 1994, a magnitude-8.3 earthquake struck about 320 km northeast of La Paz,
Bolivia, at a depth of 636 km. This earthquake, within the subduction zone between the
Nazca Plate and the South American Plate, was one of deepest and largest subduction
earthquakes recorded in South America. Fortunately, even though this powerful
earthquake was felt as far away as Minnesota and Toronto, Canada, it caused no major
damage because of its great depth.
Ring of Fire [76 k]
Oceanic-continental convergence also sustains many of the Earth's active volcanoes, such
as those in the Andes and the Cascade Range in the Pacific Northwest. The eruptive
activity is clearly associated with subduction, but scientists vigorously debate the possible
sources of magma: Is magma generated by the partial melting of the subducted oceanic
slab, or the overlying continental lithosphere, or both?
Oceanic-oceanic convergence
As with oceanic-continental convergence, when two oceanic plates converge, one is
usually subducted under the other, and in the process a trench is formed. The Marianas
Trench (paralleling the Mariana Islands), for example, marks where the fast-moving
Pacific Plate converges against the slower moving Philippine Plate. The Challenger
Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior
(nearly 11,000 m) than Mount Everest, the world's tallest mountain, rises above sea level
(about 8,854 m).
Subduction processes in oceanic-oceanic plate convergence also result in the formation of
volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the
ocean floor until a submarine volcano rises above sea level to form an island volcano.
Such volcanoes are typically strung out in chains called island arcs. As the name implies,
volcanic island arcs, which closely parallel the trenches, are generally curved. The
trenches are the key to understanding how island arcs such as the Marianas and the
Aleutian Islands have formed and why they experience numerous strong earthquakes.
Magmas that form island arcs are produced by the partial melting of the descending plate
and/or the overlying oceanic lithosphere. The descending plate also provides a source of
stress as the two plates interact, leading to frequent moderate to strong earthquakes.
Continental-continental convergence
The Himalayan mountain range dramatically demonstrates one of the most visible and
spectacular consequences of plate tectonics. When two continents meet head-on, neither
is subducted because the continental rocks are relatively light and, like two colliding
icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed
upward or sideways. The collision of India into Asia 50 million years ago caused the
Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow
continuous convergence of the two plates over millions of years pushed up the Himalayas
and the Tibetan Plateau to their present heights. Most of this growth occurred during the
past 10 million years. The Himalayas, towering as high as 8,854 m above sea level, form
the highest continental mountains in the world. Moreover, the neighboring Tibetan
Plateau, at an average elevation of about 4,600 m, is higher than all the peaks in the Alps
except for Mont Blanc and Monte Rosa, and is well above the summits of most
mountains in the United States.
Above: The collision between the Indian and Eurasian plates has pushed up the
Himalayas and the Tibetan Plateau. Below: Cartoon cross sections showing the meeting
of these two plates before and after their collision. The reference points (small squares)
show the amount of uplift of an imaginary point in the Earth's crust during this mountainbuilding process.
| The Himalayas: Two Continents Collide |
Transform boundaries
The zone between two plates sliding horizontally past one another is called a transformfault boundary, or simply a transform boundary. The concept of transform faults
originated with Canadian geophysicist J. Tuzo Wilson, who proposed that these large
faults or fracture zones connect two spreading centers (divergent plate boundaries) or,
less commonly, trenches (convergent plate boundaries). Most transform faults are found
on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag
plate margins, and are generally defined by shallow earthquakes. However, a few occur
on land, for example the San Andreas fault zone in California. This transform fault
connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda -Juan de Fuca -- Explorer Ridge, another divergent boundary to the north.
The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the many
fracture zones (transform faults) that scar the ocean floor and offset ridges (see text). The
San Andreas is one of the few transform faults exposed on land.
The San Andreas fault zone, which is about 1,300 km long and in places tens of
kilometers wide, slices through two thirds of the length of California. Along it, the
Pacific Plate has been grinding horizontally past the North American Plate for 10 million
years, at an average rate of about 5 cm/yr. Land on the west side of the fault zone (on the
Pacific Plate) is moving in a northwesterly direction relative to the land on the east side
of the fault zone (on the North American Plate).
San Andreas fault [52 k]
Oceanic fracture zones are ocean-floor valleys that horizontally offset spreading ridges;
some of these zones are hundreds to thousands of kilometers long and as much as 8 km
deep. Examples of these large scars include the Clarion, Molokai, and Pioneer fracture
zones in the Northeast Pacific off the coast of California and Mexico. These zones are
presently inactive, but the offsets of the patterns of magnetic striping provide evidence of
their previous transform-fault activity.
Plate-boundary zones
Not all plate boundaries are as simple as the main types discussed above. In some
regions, the boundaries are not well defined because the plate-movement deformation
occurring there extends over a broad belt (called a plate-boundary zone). One of these
zones marks the Mediterranean-Alpine region between the Eurasian and African Plates,
within which several smaller fragments of plates (microplates) have been recognized.
Because plate-boundary zones involve at least two large plates and one or more
microplates caught up between them, they tend to have complicated geological structures
and earthquake patterns.
Rates of motion
We can measure how fast tectonic plates are moving today, but how do scientists know
what the rates of plate movement have been over geologic time? The oceans hold one of
the key pieces to the puzzle. Because the ocean-floor magnetic striping records the flipflops in the Earth's magnetic field, scientists, knowing the approximate duration of the
reversal, can calculate the average rate of plate movement during a given time span.
These average rates of plate separations can range widely. The Arctic Ridge has the
slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter Island, in the
South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).
Easter Island monolith [80 k]
Evidence of past rates of plate movement also can be obtained from geologic mapping
studies. If a rock formation of known age -- with distinctive composition, structure, or
fossils -- mapped on one side of a plate boundary can be matched with the same
formation on the other side of the boundary, then measuring the distance that the
formation has been offset can give an estimate of the average rate of plate motion. This
simple but effective technique has been used to determine the rates of plate motion at
divergent boundaries, for example the Mid-Atlantic Ridge, and transform boundaries,
such as the San Andreas Fault.
GPS Satellite and Ground Receiver [63 k]
Current plate movement can be tracked directly by means of ground-based or spacebased geodetic measurements; geodesy is the science of the size and shape of the Earth.
Ground-based measurements are taken with conventional but very precise groundsurveying techniques, using laser-electronic instruments. However, because plate motions
are global in scale, they are best measured by satellite-based methods. The late 1970s
witnessed the rapid growth of space geodesy, a term applied to space-based techniques
for taking precise, repeated measurements of carefully chosen points on the Earth's
surface separated by hundreds to thousands of kilometers. The three most commonly
used space-geodetic techniques -- very long baseline interferometry (VLBI), satellite
laser ranging (SLR), and the Global Positioning System (GPS) -- are based on
technologies developed for military and aerospace research, notably radio astronomy and
satellite tracking.
Among the three techniques, to date the GPS has been the most useful for studying the
Earth's crustal movements. Twenty-one satellites are currently in orbit 20,000 km above
the Earth as part of the NavStar system of the U.S. Department of Defense. These
satellites continuously transmit radio signals back to Earth. To determine its precise
position on Earth (longitude, latitude, elevation), each GPS ground site must
simultaneously receive signals from at least four satellites, recording the exact time and
location of each satellite when its signal was received. By repeatedly measuring distances
between specific points, geologists can determine if there has been active movement
along faults or between plates. The separations between GPS sites are already being
measured regularly around the Pacific basin. By monitoring the interaction between the
Pacific Plate and the surrounding, largely continental plates, scientists hope to learn more
about the events building up to earthquakes and volcanic eruptions in the circum-Pacific
Ring of Fire. Space-geodetic data have already confirmed that the rates and direction of
plate movement, averaged over several years, compare well with rates and direction of
plate movement averaged over millions of years.
There are a few handfuls of major plates and dozens of smaller, or minor, plates. Six of
the majors are named for the continents embedded within them, such as the North
American, African, and Antarctic plates. Though smaller in size, the minors are no less
important when it comes to shaping the Earth. The tiny Juan de Fuca plate is largely
responsible for the volcanoes that dot the Pacific Northwest of the United States.
The plates make up Earth's outer shell, called the lithosphere. (This includes the crust
and uppermost part of the mantle.) Churning currents in the molten rocks below propel
them along like a jumble of conveyor belts in disrepair. Most geologic activity stems
from the interplay where the plates meet or divide.
The movement of the plates creates three types of tectonic boundaries: convergent,
where plates move into one another; divergent, where plates move apart; and transform,
where plates move sideways in relation to each other.
Convergent Boundaries
Where plates serving landmasses collide, the crust crumples and buckles into mountain
ranges. India and Asia crashed about 55 million years ago, slowly giving rise to the
Himalaya, the highest mountain system on Earth. As the mash-up continues, the
mountains get higher. Mount Everest, the highest point on Earth, may be a tiny bit taller
tomorrow than it is today.
These convergent boundaries also occur where a plate of ocean dives, in a process called
subduction, under a landmass. As the overlying plate lifts up, it also forms mountain
ranges. In addition, the diving plate melts and is often spewed out in volcanic eruptions
such as those that formed some of the mountains in the Andes of South America.
At ocean-ocean convergences, one plate usually dives beneath the other, forming deep
trenches like the Mariana Trench in the North Pacific Ocean, the deepest point on Earth.
These types of collisions can also lead to underwater volcanoes that eventually build up
into island arcs like Japan.
Divergent Boundaries
At divergent boundaries in the oceans, magma from deep in the Earth's mantle rises
toward the surface and pushes apart two or more plates. Mountains and volcanoes rise
along the seam. The process renews the ocean floor and widens the giant basins. A single
mid-ocean ridge system connects the world's oceans, making the ridge the longest
mountain range in the world.
On land, giant troughs such as the Great Rift Valley in Africa form where plates are
tugged apart. If the plates there continue to diverge, millions of years from now eastern
Africa will split from the continent to form a new landmass. A mid-ocean ridge would
then mark the boundary between the plates.
Transform Boundaries
The San Andreas Fault in California is an example of a transform boundary, where two
plates grind past each other along what are called strike-slip faults. These boundaries
don't produce spectacular features like mountains or oceans, but the halting motion often
triggers large earthquakes, such as the 1906 one that devastated San Francisco.
Volcanoes of Iceland:
Iceland, the land of ice and fire, is a true paradise for volcanologists. In few places on
earth, geology and human history are so closely connected to volcanism as on Iceland.
The island owns its existence to a large volcanic hot spot sitting on a mid-oceanic ridge, a
unique setting. The plate boundary between the American and Eurasian tectonic plates
crosses Iceland from south to North and the spreading process can be directly measured
and observed on land.
The Creation of the Andes
The Andes were created when the submarine Pacific Ocean plates pushed against the
American continental plate. The pressure resulted in sierras. Sierras are defined as a
mountain range that has a irregular, toothed contour. Sierras are produced when the
sedimentary rocks break and pyroclastic materials, including molten granite and igneous
rocks, bursts out from below. At this same time, volcanoes on the oceanic plates
exploded. To this day volcano and earthquake eruptions are very common.
As a result of all the pushing and the pressure, the complex mountain range was birthed.
There are two major chains of mountains with a very deep depression and out of the
major chains, many minor chains spurt out.
There has actually been a theory that the Andes Mountains were once a part of a larger
mountain range including it and the Rocky Mountains. But after time, plates moved,
continents moved, and the mountain range has been separated.
The Ring of Fire
In the not too distant past, cartographers charted the deep ocean trenches, seismologists
plotted earthquakes beneath the trenches, and volcanologists studied the overlying
volcanoes. But these researchers worked independently of each other, and were unaware
that the phenomena they studied were all part of a singular process. Today, the ideas of
sea-floor spreading and subduction explain clearly why so many of the world’s volcanoes
are situated on the Pacific island arcs, the Ring of Fire, where earth’s tectonic plates are
being subducted beneath deep ocean trenches.
After about 10 million years, the final stage of subduction begins. At depths of as much
as 450 miles, the plate becomes so hot that it softens and stops generating earthquakes.
But the descent and melting continue until, at some unknown depth, the plate blends with
the surrounding mantle material. Eventually this material will emerge along Mid-Ocean
Ridges as new sea-floor crust or escape as volcanic lava as the process of subduction
comes full circle and our tectonic planet continues to evolve.
Mountain Ranges While new ocean crust is constantly being created at mid-ocean ridges,
old crust must either be destroyed or reduced at the same rate (or else the planet would be
continually expanding and increasing in volume). The plates, therefore, emerging along
mid-ocean ridges, sliding over the athenosphere, and grinding past other plates along
transform faults, are almost all headed on a collision course. When two continents carried
on converging plates ram into each other, they crumple and fold under the enormous
pressure, creating great mountain ranges.
The highest mountain range in the world, the snow-capped Himalayas, is an example of a
continent-to-continent collision. This immense mountain range began to form when two
large landmasses, India and Eurasia, driven by tectonic plate movement, collided.
Because both landmasses have about the
same rock density, one plate could not be
subducted under the other. The pressure
of the colliding plates could only be
relieved by thrusting skyward. The
folding, bending, and twisting of the the
collision zone formed the jagged
Himalayan peaks. This string of
towering peaks is still being thrust up as
India, embedded in the Indo-Australian
Plate, continues to crunch relentlessly into Tibet, on the southern edge
of the Eurasian Plate.
Here's a more detailed chronological explanation.
About 220 million years ago, India was an island situated off the
Australian coast, and separated from the Asian continent by a vast
ocean called the Tethys Sea. When Pangaea broke apart about 200
million years ago, India began to move northward. Scientists have
been able to reconstruct India's northward journey. When India rammed into Asia about
50 million years ago, its northward advance slowed. The collision and decrease in the rate
of plate movement mark the beginning of the Himalayan uplift.
Fossilized Sea Shells near Himalayan Peaks?
When archaeologists found the fossilized remains of ancient sea-creatures near the peaks
of the Himalayas they were, understandably, puzzled. Intriguing questions were raised.
Was there once an ocean or other large body of water at the top of this enormous
mountain range? Unlikely.
Had the entire planet, Himalayas and all, at some point in Earth’s long history, been
submerged underwater? Possibly - but highly improbable. No theory could fully explain
this apparent paradox. Until the theory of plate tectonics was put forth.
Briefly, it goes like this: As the Indo-Australian Plate, with India firmly embedded,
approached the Eurasian continent 20 million years ago, its leading edge, comprised of
oceanic crust, was first to be crumpled and uplifted. Slowly, the Himalayas rose and the
leading oceanic crust of the Indian sub-continent, carrying the fossilized remains of its
ancient ocean inhabitants, was thrust up by the crumpling crust in its wake. Thus, plate
tectonics explains how the majestic peaks of one of the world’s great mountain ranges
were once the deep sea-floors of an ancient drifting plate.
The European Alps have been formed in similar fashion, starting some 80 million years
ago when the outlying continental fragments of the African Plate collided with the
Eurasian Plate. Unyielding pressure between the two plates continues even today,
resulting in the gradual closing up of the Mediterranean Sea.
Growing Mountains
As an underlying oceanic plate tips down, its ocean-floor sediment is scraped off along
the front edge of the overriding continental plate. The result is an increase in the width
and thickness of the overriding plate. This could be why the Andes, a long mountain
range bordering the west coast of South America, appears to be growing higher. Perhaps
sediment from the Nazca Plate, which is diving under South America in the Peru-Chile
Trench, is scraping off on the roots of the Andes. This scraping adds thickness and
buoyancy to the mountains so that they float upward more rapidly than their peaks can be
eroded by wind and rain.
Questions –
1. What is the difference between major and minor plates? Do the major plates have
more impact on the Earth?
2. What is the lithosphere?
3. What is a convergent plate, a divergent plate, and a transform boundary?
4. What happens when plates that have land on them collide?
5. How were the Himalaya mountains created?
6. What is subduction?
7. What is a trench?
8. How was the Great Rift Valley in Africa formed?
9. What does a transform boundary usually cause?
10. How was Iceland made?
11. How were the Andes mountains made?
12. Scientists think that the Andes were once linked with what other mountain range?
13. What is the ring of fire?
14. What explains why so many of the world’s volcanoes are situated on the Pacific
island arcs?
15. After the plate bends and melts, where will it emerge? What will it become?
16. How do we know that old crust is being destroyed or reduced at the same rate as new
crust is being made?
17. Where was India 220 million years ago?
18. Were the Himalaya mountains ever under water? Explain your answer.
19. Why do the Andes mountains appear to be growing higher?
20. Write down 3 questions have after reading these articles.
Questions for USGS
1. What are the 4 types of boundaries, and briefly describe each.
2. What determines the “type of collision” between plates?
3. Where was the largest subduction earthquake in South America?
4. What is a triple junction, and give an example of one.
5. What are trenches, and how are they formed?
6. Where are most transform faults located?
7. Why is Iceland a great natural laboratory for plate tectonics?
8. How were the Andes mountains formed? What commonly takes place
there?
9. How can scientists measure the speed of tectonic plates?
10. Where does the magma come from for the active volcanoes in the ring
of fire? (scientists argue about 2 different sources)
11. Why has the size of Earth not changed significantly over the past 600
million years?
12. What is the most useful tool for studying Earth’s crustal movements and
why?
13. Make a metaphor for a divergent boundary.
14. How much does the land move in the San Andreas fault per year?
15. What are island arcs?
16. What is the Mid-Atlantic Ridge? What type of boundary is it?
17. What is the Mariana Trench?
18. How was the Red Sea formed?
19. How did the Himalayas form? When?
20. Where do scientists think the next major ocean will form, and why?